One of the most important steps in studying chemical reaction kinetics is to determine a rate law for the reaction. In this paper, a method for the identification of the reaction kinetic model using isothermal calorimetric data and additional measurement of the rate of pressure change in a constant volume of a calorimetric vessel is described. This method can be applied to fairly fast chemical reactions (i.e. those that can be completed within about 2 h) that produce gaseous products. The decomposition of formic acid in the heterogeneous Fenton process is employed as a case study to demonstrate the use of the developed method for the kinetic analysis of a complex reaction system. It was found that the rate of decomposition of hydrogen peroxide can be described by the Langmuir-Hinshelwood type of equation in the form r = kC(p)/(1 KpCp) (where C-p is a molar concentration of H2O2) both in the case of the decomposition of the aqueous hydrogen peroxide solution as well as during the oxidation of formic acid. A simple, purely empirical rate equation for the decomposition of formic acid was also proposed.